A kinetic study of the formation of ketimine hydrochlorides. The mechanism of the Houben–Hoesch reaction

Abstract

We report a kinetic study of ketimine hydrochloride formation from phloroglucinol, hydrogen chloride, and both aceto- and benzo-nitrile. The reaction rate is of the first-order in phloroglucinol and in the nitrile. In the general case it is of variable order in hydrogen chloride, but this order becomes unity in special circumstances. With zinc chloride as catalyst the order in this component is normally unity (or close to it). The aromatic substrate undergoes appeciable protonation in the reaction mixtures, especially those containing zinc chloride, and the protonated species does not undergo electrophilic attack. A novel mechanism is suggested which rationalises a wide range of data.

The stability and acidity of solutions of hydrogen and zinc chloride in nitriles is discussed. It is suggested that in the hydrogen chloride systems anions [Cl(HCl)_n]^– exist with n > 1.